“You know,” she said, at length, “there’ll have to be some sort of explanation.”

“Well?” demanded the Professor.

“If I knew what you had done to Mr. Smith,” she went on, “I could help you to keep things as quiet as possible.”

He heard her with a frown and shook his head.  “If you knew, you’d do anything but keep it quiet,” he answered shortly.

“Then it was something horrible?” asked Mary quickly.

He smiled.  “I expect to have many patients for the same treatment,” he replied.  “Very many; I expect half the world.  Where is Smith now?” he asked abruptly.

“At home by himself,” replied Mary.  “We’ll be there in two minutes.  You’d like to see him first?”

“Yes, please,” he said.  “I must have a word or two with him.”

Dr Pond had not returned when they drew up at the house, and, as soon as the Professor had rid himself of his ulster and hat, she led him upstairs to the “study.”

“You’ll find him in here,” she said, when they came to the door.  “I shall be downstairs when you want me.”

The Professor nodded absently and turned the handle.  Mary was at the top of the stairs when he entered.  She turned even before he cried out, conscious of something happening.

“Stop!” cried the Professor sharply.  “Put that down!”

Mary ran to the open door and uttered a cry.  Near the window stood Smith, erect and buoyant.  The contents of desk-drawers were littered on the floor—­papers, old pipes, a corkscrew, various rubbish—­and in his hand he held something that Mary recognized with a catch of the breath.

“Father’s old pistol!” she said, and shuddered.  The Professor had advanced as far as the middle of the room; the desk was between him and Smith, who was looking at him with a smile.  Even in the weakness of fear that came over her, Mary wondered at the change in him.  His very stature seemed to be greater; there was a grave power in that face she knew as a mask of witlessness and futility.  He held the revolver in his right hand with the barrel resting in his left, and looked at the tall Professor with a smile that had no mirth in it, but something like compassion.

“Drop it!” said the Professor again.  “Drop it, you fool!” But his voice of authority cracked, and he cried out:  “For God’s sake don’t make a mess of it now.”

Smith continued to look at him with that ghost of a smile on his lips, and answered with slow words.  He patted the pistol.

“This’ll put me out of your reach,” he said.  “This is what’ll do it.  You won’t be able to patch up the hole this’ll make.”

He raised the pistol, Mary, powerless to move clenched her hands and whole being for the shock of imminent tragedy.